1. Field of the Invention
The present invention relates to a watercraft propulsion device.
2. Description of the Related Art
There is a known control to suppress a rotational speed of an engine by stopping a spark ignition (hereinafter called “misfiring”) when a rotational speed of the engine exceeds a prescribed rotational speed. For example, an ignition control apparatus of an engine disclosed in Laid-open Japanese Patent Application Publication No. 2002-480412 is configured to set a misfiring mode when a throttle is opened rapidly. While the misfiring mode is set, misfiring of the engine is executed when the engine rotational speed becomes equal to or higher than a prescribed rotational speed. Meanwhile, Laid-open Japanese Patent Application Publication No. 03-210068 discloses an engine control apparatus in which a temperature of an engine is detected using a temperature sensor and a rotational speed of the engine is detected using a rotational speed sensor. If engine overheating is detected, then the apparatus executes misfiring of the engine according to a prescribed procedure such that the rotational speed of the engine becomes equal to a prescribed rotational speed.
With the engine ignition control apparatus disclosed in Laid-open Japanese Patent Application Publication No. 2002-480412, an engine can be prevented from being over-revved due to a rapid operation of a throttle by a driver. However, in the case of an outboard boat motor or other watercraft propulsion device, there are times when the engine enters an unloaded state temporarily due to, for example, the propeller becoming exposed above the water surface. In such a case, the engine rotational speed may increase abruptly even though the operator has not performed an abrupt operation of the throttle. Thus, in the case of a watercraft propulsion device, it is difficult for an increase of the engine rotational speed to be suppressed using the control presented in Laid-open Japanese Patent Application Publication No. 2002-480412.
In the engine control apparatus disclosed in Laid-open Japanese Patent Application Publication No. 03-210068, the engine rotational speed is controlled to a prescribed rotational speed by executing engine misfiring according to a prescribed procedure. Consequently, an increase of the engine rotational speed can be suppressed regardless of whether an operator performs an abrupt throttle operation or not. However, if the engine enters an unloaded state as explained above, then the engine rotational speed will increase instantaneously. It is difficult to counter such an instantaneous increase of the engine rotational speed in an instantaneous fashion by merely executing misfiring in accordance with the engine rotational speed in the manner of the engine control apparatus disclosed in Laid-open Japanese Patent Application Publication No. 03-210068.